1996 850 - Can a new fuel filter change idle speed????

2 weeks ago changed the fuel filter on my wife's 855. Bosch part,
looked identical to the old Volvo one on the car. Total Mileage
80,000. Just drove it this weekend and noticed that idle in gear at
rest is 1000 rpm, previously was about 850rpm. At rest in neutral
(autobox) idle is 1300rpm, previously 900rpm. Car runs well, just did
200mile trip, all fine, gears change fine, engine smooth etc. Air
filter, plugs, oil filter all replaced in last 3000miles (oem parts).
Throttle body and flame trap cleaned 3000 miles ago (they were not
very dirty at all). I checked for vacuum hose missing or broken and
could not see anything adrift.I can't believe changing a fuel filter
would affect the idle. Also, I can't see that I installed the filter
incorrectly. It does not leak and the engine runs so surely its OK. Is
it just a coincidence and maybe the Idle Control Unit is playing up?
Any suggestions gratefully received

Chris

 

This evening I removed the idle control unit. The valve inside was
moving quite smoothly but I cleaned it up anyway. The electrical
connectors had a small amount of turquoise corrosion. I cleaned these
up also and put it all back together. Only took 20 mins. When I
started the engine it immeditely settled into a smooth idle at 900rpm
in Park. Not possible to do a test drive so will wait until tomorrow.
Could the slight corrosion on the connector hav caused my problem, I
wonder.

 

Car is now idling smoothly at 900rpm in neutral. So it must have been
either the corrosion on the electrical connectors or sticking valve in
idle control unit

 

Another possibility is that the engine computer recalibrated to the better
fuel flow of the new filter. This is a possibility if the fuel filter was
very clogged and would fit with 80,000 mileage on the same filter.

 

While your suggestion is not beyond the realm of physical possibility,
any car that had it's idle speed largely dependent on the rate of fuel
flow from the tank would be really scary.

 

Update - at the weekend the high idle returned after we had been
driving for about 5 km. I restarted the engine and it went back to
900rpm. I am now concerned that it is something electronic - would
turnining the ignition off and restarting re-set something. Note that
the Engine Check light is not illuminated. Something else I should add
- the ABS Control Unit is out of the car for repair, but I can't see
this would have any impact.

 

Chris, I believe your computer had come to misread things due to the
corrosion you removed. By reinstalling it and simply restarting, the
computer said, "Hey something is different. Let me check this out."
After a short ride, it found where truth was and reset.

Interesting tip on a simple fix. Wanna bet the dealer had another,
higher price solution? I also tend to think that an independent
mechanic practiced in Volvos would have also come to your solution. He
doesn't have quite the same reinforcement contingencies a dealer has.

Chuck Fiedler
Nothing but Volvo since 1974
Oh, yeah, I finally got rid of my 1960 Austin Healey

 

Further update. It is still idling irregularly. Sometimes 900rpm, then
it goes to 1400rpm. My initial view that it was the idle control valve
connections now appears prematurely optimistic. It has been suggested
to me that the front o2 sensor could be playing up. The car has
81,000miles on original o2 sensors. Could this cause a high idle? What
makes me a bit dubious though is the the check engine light is not
illuminated. I would have thought that a bad o2 sensor would have been
detected by the EMS and reported? Given the mileage, I am tempted to
replace the front o2 sensor as a matter of course as I understand they
have a max life or around 100,000miles anyway, and it seems like a 30
minute job. Any thoughts?

 

I would give that a try. Actually, *I* would take it to my mechanic
who has all the diagnostic stuff and have him fix it but since you're
shooting in the dark, give that a try. It certainly can't hurt and
self-repair is cheaper than paying for it. Self-diagnosis OTOH, can be
rather expensive in the long run.

 

While the O2 sensor may be weak (Bosch recommends 60,000 mile replacement
intervals. But keep in mind they are the primary manufacturer of this
costly product), I don't think it is the source of your problem. I agree
that a substantial O2 sensor problem would trigger the check engine light if
that was the primary cause. Also, the O2 sensor affects fuel mixture (and
fuel economy) more than having to do with erratic idle speed. From your
latest update/description, it is more likely that you have a
cracked/intermittently disconnected vacuum line somewhere that is playing
havoc with your idle speed. This is something you can check yourself
because parts are very cheap (snip the end off a hose and reconnect to
verify the problem), but very labor intensive. Professionals charge dearly
for their time and don't always do a thorough job when it is this boring.
Don't confuse/misconnect the hoses. That is when the professionals charge
the really high labor rate to correct an amateur's mistake.

 

Walt Kienzle?! Is this the Walt Kienzle who used ot be in Minneapolis
who I would take my 544 to for miraculous cures?

If so, where are you now?

Please email me with particulars.

Chuck Fiedler
Nothing but Volvo since 1974

 

it is more likely that you have a

I did a quick check of all the vacuum hoses when the problem first
mainfested - could not see enything wrong, but I shall do a more
thorough check as you suggest. Even though it is 12 yrs old all the
hoses were surprisingly supple. They did not seem to have hardened/
cracked with age and heat. Thanks

 

Sorry, not the same Walt Kienzle (there are two of me??).

 

I don't know if there are two of you but I knew a Walt Kienzle thirty
years ago and my father did his bookwork. He was a wizard at fixing
Volvos. I heard he had moved to the Dallas area long after i had.

I sorta figgered there are a limited number of folks named Walt
Kienzle and if you filter it through Volvo, you'd get a pretty small
set of hits.

Sorry if I hit the wrong guy but glad to make your acquaintance.

Chuck Fiedler
Nothing but Volvo since 1974